Receiver Having an Adaptive Filter and Method of Optimizing the Filter

ABSTRACT

A receiver comprises an adaptive filter having an input for a digitized input signal, means for storing a pre-designed filter characteristic, means for analyzing a digital. representation of the input signal to determine a desired position of the filter characteristic to match the system requirements, and means for adapting the stored pre-designed filter characteristic in the frequency domain and/or the time domain to match the system requirements and for transforming the adapted filter characteristic to the time domain to update coefficients for the adaptive filter and for loading updated coefficients into adaptive filter. The updating of the coefficients may be done periodically. The adaptation may be one or more of adjusting bandwidth, frequency shift and, in the case of a bandpass characteristic, superimposing characteristics.

TECHNICAL FIELD

The present invention relates to a receiver having an adaptive filterand to a method of adapting and optimizing the characteristics of theadaptive filter. The receiver has particular, but not exclusive,application to receiving broadband OFDM/CDMA signals in the ISM band.

BACKGROUND INFORMATION

Many receivers use some form of digital filtering for a variety ofpurposes including channel selection, channel rejection and interferencerejection. The specific filtering requirements for individual scenariosare generally dynamic, for example for channel or interferencerejection, and in these situations a dynamic filter allows optimumperformance, for example how well an interferer is rejected, for theleast complexity and/or power consumption.

In a broadband OFDM/CDMA system operating in the ISM band there are manysources of interference, one of which is narrowband frequency hoppingsystems. Adaptive filters can be used in CDMA applications where CDMAsignals are interfered with by a narrowband jammer. In an article“Adaptive Digital Signal Processing JAVA Teaching Tool” by M. Hartneckand R. W. Stewart, submitted to IEEE Transactions on Education-SpecialCDROM Issue, November 1999, also available on the internet at:http://www.spd.eee.strath.ac.uklusers/bob/adaptivejava/begin.htm, thereis disclosed an example of CDMA interference suppression in which if abroadband (stochastic) signal has interference from a narrowband(periodic) source, a prediction architecture can be used to attempt tofind correlation between an output y(k) of an adaptive filter and aninput signal which has been fed forward from a delayed input of theadaptive filter. By taking the difference between the signals, viz.d(k)−y(k), the narrowband signal is attenuated and it is found that anoutput signal e(k) is approximately equal to the signal applied by adata source to the transmission channel. As a generality, adaptivefilters use error calculations in order to make minor adjustments to thefilter coefficients. As the demands for high performance filtering growthere is an attendant problem of complexity and increased powerconsumption.

BRIEF SUMMARY

An object of the present invention is to provide an adaptive filterwhich can achieve a high performance coupled with a less complexstructure and a lower power consumption than known adaptive filters.

According to one aspect of the present invention there is provided amethod of dynamically adapting a digital filter characteristic,comprising storing a predetermined frequency representation of thefilter, analyzing an input signal, adapting the filter characteristic tomatch the system requirements, transforming a frequency domainrepresentation of the adapted filter characteristic to the time domain,and calculating new filter coefficients to effect the adaption of thefilter characteristics.

According to a second aspect of the present invention there is provideda receiver comprising an adaptive filter having an input for a digitizedinput signal, means for storing a pre-designed filter characteristic,means for analyzing a digital representation of the input signal todetermine a desired position of the filter characteristic to match thesystem requirements, means for adapting the stored pre-designed filtercharacteristic to match the system requirements, and means fortransforming the adapted filter characteristic to the time domain toupdate coefficients for the adaptive filter and for loading updatedcoefficients into the adaptive filter.

The adaptation of the filter characteristic may be effected in thefrequency domain, for example by moving filter taps to the left or rightand then doing an IFFT, in the time domain, for example by multiplyingall time domain taps by a sine wave of the desired shift frequency, orin a combination of both frequency and time domains.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, wherein:

FIG. 1 is a block schematic diagram of a receiver made in accordancewith the present invention,

FIG. 2 is a flow chart relating to a method of adapting and optimizingthe characteristics of a digital filter used in the receiver shown inFIG. 1,

FIGS. 3, 4 and 5 respectively show a bandstop filter characteristicstored in a memory of the receiver and the same characteristic shiftedto the left and to the right of the position shown in FIG. 3,

FIGS. 6 and 7 respectively show a widened version of the originalbandstop filter characteristic and the same characteristic shifted tothe right of the position shown in FIG. 6, and

FIGS. 8 to 13 show a number of bandpass filter characteristics.

DETAILED DESCRIPTION

In the drawings the same reference numerals have been used to indicatecorresponding features.

Referring to FIG. 1, the receiver comprises an antenna 10 connected byway of a RF amplifier 12 to a first input of a mixer 14. A localoscillator 16 for mixing the received signal down to baseband is coupledto a second input of the mixer 14. A low pass filter 18 selects thewanted products of mixing from the signals at the output of the mixer14. An analog-to-digital converter (ADC) 20 which may be implemented asa sigma delta modulator is coupled to the low pass filter 18. A FIRfilter 22 which may be implemented as a field programmable gate array,an application specific integrated circuit (asic) or a Digital SignalProcessor (DSP) with FIR filter is coupled to an output of the ADC 20. ADSP 24 is coupled to the output of the FIR filter 22 in order to analyzethe received signal and to adapt the filter characteristics accordingly.In the illustrated embodiment of the DSP 24 it comprises a first block26 which serves to analyze the input signal, that is, to find theposition of interference and its severity. A second stage 28 manipulatesthe original FIR filter in the frequency domain and converts it from thefrequency domain to the time domain to obtain the FIR coefficients.

A frequency domain version of the original FIR filter is stored in amemory 30 which is coupled to the second stage 28. The second stage 28is coupled by a line 32 to the FIR filter 22 to enable new coefficientsto be loaded with the FIR filter 22. The calculation and loading of newcoefficients may be periodic, for example once every N communicationframes to reduce the burden on the DSP 24.

In operation of the receiver, a pre-designed frequency representation ofthe filter is stored in the memory 30 and the characteristics areadapted as a result of analyzing the input signal in the first stage 26to match those required by the system. The adaptation of the filtercharacteristic can be effected (a) in the frequency domain by shiftingfrequency domain filter taps left or right in the frequency domain andthen doing an IFFT, (b) in the time domain by multiplying time domainfilter taps by a sine wave of the required frequency, or (c) acombination of both by initially adapting the characteristic in thefrequency domain and manipulating the characteristic further in the timedomain.

Although the frequency domain and the time domain methods areequivalent, the frequency domain method has an inherent granularity,that is the frequency of the filter can be shifted by, for example (100,200, 300, 1300, 1400 . . . N * 100) Hz, whereas the time domain methodenables a precise frequency shift of say 1 MHz to be effected.

Once adapted, the frequency domain representation of the filter istransformed back to the time domain in order to obtain the newcoefficients or tap weightings which are loaded into the FIR filter 22by way of the line 32.

Before describing the flow chart in FIG. 2 the understanding of theprocess will be better understood by considering FIGS. 3 to 5. In thesefigures, the abscissa represents frequency and the ordinate attenuationof the bandstop filter having 104 taps. FIG. 3 shows the pre-designedfrequency representation of the FIR filter 22 having a notch 34dimensioned to block out a narrowband interferer. The filtercharacteristic is stored in the memory 30. Such a filter is useful for abroadband of OFDM/CDMA system which is operating in the ISM band. In theISM band one of the sources of interference is narrowband frequencyhopping systems. In order to block out a frequency hopping interferer itis necessary to position the filter characteristic wherever necessary sothat the notch 34 can block out this narrowband signal.

In operation the DSP 24 determines the position of the interferer andmanipulates the filter 22 so that the notch 34 is shifted to block theinterferer. FIGS. 4 and 5 show different positions to which the notch 34has been shifted whilst leaving the shape of the notch unaltered.

In many cases the receiver can predict where the interferer willfrequency hop to because the hopping algorithms are known and cantherefore act pro-actively rather than reactively and in so doing, makefurther performance gains.

Referring to FIG. 2, the flow chart begins with a block 40 which relatesto the process of designing a FIR filter using a filter design package.Block 42 relates to the process of transforming the impulse response tothe frequency domain. Block 44 relates to permanently storing thefrequency domain samples in the memory 30 of the receiver. Block 46relates to the receiver measuring the required filter characteristics byanalyzing the received signal. Block 48 denotes the receiver adaptingthe characteristics of the stored filter to match those required. Thecharacteristics which may be altered are (1) bandwidth which is adjustedby reducing or increasing the number of samples in the stored frequencydomain characteristic; (2) frequency shift which in the frequency domainis adjusted by shifting the samples of the stored frequency domaincharacteristic left or right or in the time domain by multiplying timedomain filter taps using a sine wave of the desired frequency; and (3)superimposed characteristics, which as will be described later, appliesonly to a bandpass filter, and which is realized by adding togetherindividual frequency domain characteristics.

In block 50 a check is made to see if bandwidth has to be altered, andif so (Y) then block 52 denotes adjusting the bandwidth.

In block 54 a check is made to see if a frequency shift is required andif so (Y) then in block 56 the frequency is shifted.

In block 58 a check is made to see if characteristics are to besuperimposed and if so (Y) then this is carried out in block 60.

A negative output (N) from each of the blocks 50, 54 and 58 is suppliedtogether with outputs from the blocks 52, 56, 60 to a block 62 whichdenotes transforming the adjusted frequency domain representation backto the time domain using a FFT which is equal in size to the number ofsample points. Block 64 relates to the receiver updating the FIR filterscoefficients with the result from the block 62. The new coefficients maybe calculated continuously or periodically, for example once every Ncommunication frames.

The flow chart thereafter returns to the block 46 whenever an update isrequired.

Referring now to FIGS. 6 and 7, the bandstop filter is a widened versionof the original filter which is less complex and less power hungry,having only 60 taps compared to 104 taps in FIGS. 3, 4 and 5. FIG. 7shows the notch 34 shifted to the right from the position shown in FIG.6.

The teachings of the present invention can be applied to a bandpassfilter having application to purposes such as channel selection in say abase station.

Referring to FIGS. 8 to 13, the abscissa represents frequency and theordinate represents power.

FIG. 8 shows an original 104 tap filter designed using a filter designerprogram. The passband is shown by the passbands 68.

FIG. 9 illustrates the filter characteristic of FIG. 8 which has beenshifted in frequency so that the passband 68 selects the desired correctchannel.

FIG. 10 shows the filter characteristic of a 104 tap filter which isformed by two superimposed versions which provide two passbands 68A, 68Ballowing two channels to pass.

FIG. 11 illustrates the filter characteristic of a 104 tap filter formedby arranging the passbands 68A, 68B adjacent to provide a wider channelhaving a high roll off.

FIG. 12 illustrates a filter characteristic of a 19 tap filter in whichthe passband 70 has been stretched compared to those of FIGS. 8 to 11and has a low rolloff.

Lastly, FIG. 13 illustrates a filter characteristic of a 142 tap filterin which the passband 72 is narrowed compared to that of FIG. 9.

In the present specification and claims the word “a” and “an” precedingan element does not exclude the presence of a plurality of suchelements. Further, the word “comprising” does not exclude the presenceof other elements or steps than those listed.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the design, manufacture anduse of receivers having adaptive filters and component parts thereforand which may be used instead of or in addition to features alreadydescribed herein.

1. A method of dynamically adapting a digital filter characteristic, themethod comprising: storing a frequency domain representation of thefilter characteristic; analyzing an input signal to determine a requiredfilter characteristic; adapting the frequency domain representation tomatch the required filter characteristic; transforming the adaptedfrequency domain representation to a time domain so as to obtain filtercoefficients relating to the required filter characteristic; andapplying the filter coefficients to effect adaptation of the filtercharacteristic.
 2. The method as claimed in claim 1, wherein adaptingthe frequency domain representation to match the required filtercharacteristic includes altering a bandwidth of the frequency domainrepresentation.
 3. The method as claimed in claim 2, wherein alteringthe bandwidth of the frequency domain representation comprises reducingor increasing a number of samples in the frequency domainrepresentation.
 4. The method as claimed in claim 1, wherein adaptingthe frequency domain representation to match the required filtercharacteristic includes altering a frequency of the frequency domainrepresentation.
 5. The method as claimed in claim 4, wherein alteringthe frequency of the frequency domain representation comprises shiftingsamples of the frequency domain representation to a higher or lowerfrequency.
 6. The method as claimed in claim 1, wherein adapting thefrequency domain representation to match the required filtercharacteristic includes superimposing individual filter characteristics.7. The method as claimed in claim 1, wherein the required filtercharacteristic includes a bandstop filter characteristic.
 8. The methodas claimed in claim 7, wherein adapting the frequency domainrepresentation includes shifting the bandstop filter characteristic toblock an interfering signal.
 9. The method as claimed in claim 8,wherein when the interfering signal is a narrowband frequency hoppingsignal, a hopping sequence is prestored to facilitate the adapting ofthe frequency domain representation.
 10. The method as claimed in claim1, wherein the required filter characteristic includes a bandpass filtercharacteristic.
 11. The method as claimed in claim 10, wherein adaptingthe frequency domain representation includes shifting the bandpassfilter characteristic to select a required channel.
 12. The method asclaimed in claim 1, wherein the filter is further adapted in the timedomain.
 13. The method as claimed in claim 1, wherein the filtercoefficients are periodically calculated.
 14. A receiver comprising: amemory device configured to store a frequency domain representation of afilter characteristic; and a processor configured to analyze an inputsignal to determine a required filter characteristic, to adapt thefrequency domain representation to match the required filtercharacteristic, to transform the adapted frequency domain representationto a time domain so as to obtain filter coefficients relating to therequired filter characteristic, and to apply the filter coefficients toeffect adaptation of the filter characteristic of an adaptive filter inthe receiver.
 15. The receiver as claimed in claim 14, wherein theprocessor is further configured to adapt the frequency domainrepresentation to match the required filter characteristic by altering abandwidth of the frequency domain representation.
 16. The receiver asclaimed in claim 15, wherein the processor is further configured toalter the bandwidth of the frequency domain representation by reducingor increasing a number of samples in the frequency domainrepresentation.
 17. The receiver as claimed in claim 14, wherein theprocessor is further configured to adapt the frequency domainrepresentation to match the required filter characteristic by altering afrequency of the frequency domain representation.
 18. The receiver asclaimed in claim 17, wherein the processor is further configured toalter the frequency of the frequency domain representation by shiftingsamples of the frequency domain representation to a higher or lowerfrequency.
 19. The receiver as claimed in claim 14, wherein theprocessor is further configured to adapt the frequency domainrepresentation to match the required filter characteristic bysuperimposing individual filter characteristics.
 20. The receiver asclaimed in claim 14, wherein the required filter characteristic includesa bandstop filter characteristic.
 21. The receiver as claimed in claim20, wherein the processor is further configured to adapt the frequencydomain representation by shifting the bandstop filter characteristic toblock an interfering signal.
 22. The receiver as claimed in claim 21,wherein when the interfering signal is a narrowband frequency hoppingsignal, a hopping sequence is prestored to facilitate the adapting ofthe frequency domain representation.
 23. The receiver as claimed inclaim 14, wherein the required filter characteristic includes a bandpassfilter characteristic.
 24. The receiver as claimed in claim 23, whereinthe processor is further configured to adapt the frequency domainrepresentation by shifting the bandpass filter characteristic to selecta required channel.
 25. The receiver as claimed in claim 14, wherein theprocessor is further configured to adapt in the time domain.
 26. Thereceiver as claimed in claim 14, wherein the processor is furtherconfigured to periodically calculate the filter coefficients.